Non-linear optical devices and materials therefor

ABSTRACT

Materials suitable for use in non-linear optical devices comprise morpholinium or thiomorpholinium salts of hydroxy-substituted aliphatic or aromatic carboxylic acids selected from tartaric acid and hydroxy-substituted benzoic and cinnamic acids, which salts have a non-centrosymmetric crystalline form. Preferred salts include morpholinium 3-hydroxybenzoate, morpholinium 4-hydroxybenzoate, morpholinium 3,5-dihydroxybenzoate, morpholinium 3-fluoro-4-hydroxybenzoate, dimorpholinium 2,3,5,6-tetrafluoro-4-hydroxybenzoate, morpholinium 4-(4-hydroxyphenyl)benzoate, morpholinium tartrate, morpholinium 4-hydroxycinnamate, thiomorpholinium 4-hydroxy-benzoate and thiomorpholinium 3,5-hydroxybenzoate.

This invention relates to certain novel second harmonic generatormaterials which produce a second harmonic generation (SHG) of opticalwavelength electromagnetic radiation, in particular laser radiation, andto non-linear optical (NLO) devices incorporating such materials.

It is known that various organic and inorganic compounds possess theability to double the frequency of laser light passing through them.This ability is known as second harmonic generation (SHG) and isparticularly significant because it provides the ability to producelaser light of higher energy than that provided by the initial laserlight source.

Known inorganic compounds which possess SHG properties includealpha-silica, potassium dihydrogenphosphate (KDP), zinc blende wurtzite,and gadolinium and terbidium molybdates. Known organic compounds whichpossess SHG properties include urea, cadmium-thiourea complexes,L-argininium dihydrogen phosphate monohydrate (LAP), some siloxane andsilicone polymeric liquid crystals, stilbene-containing liquid crystals,some silver containing emulsions, dipotassium tartrate hemihydrate,potassium sodium tartrate tetrahydrate, compounds having large secondarymolecular susceptibilities (beta-values) such as4-(N-pyrrolidino)-3-(N-ethanamido)-nitrobenzene (PAN) and4-(dimethylamino)-3-(N-ethanamido)-nitrobenzene (DAN), and blends oflarge beta-value compounds with polypeptides as disclosed in EuropeanPatent Application Number EP-0338702-A1. A further group of materialshaving SHG properties is described in applicant's U.S. Pat. No.5,352,388 and comprise a salt of an organic nitrogenous base with anoptically active enantiomer of a chiral carboxylic acid, said acidcontaining, in addition to its carboxyl group, at least one substituentgroup selected from carboxyl and hydroxyl and the salt having anon-centrosymmetric crystal structure.

Within the field of known SHG materials, crystalline materials form animportant class because many can be grown from solution into large,transparent single crystals of good optical quality. However, ideally acrystalline SHG material must possess a combination of desirableproperties to be practically useful for incorporation in an NLO device.Amongst the most important of these properties are:—

-   -   (1) High solubility in organic and/or aqueous media in order to        promote a reasonable rate of crystal growth from solution.    -   (2) Good crystal growth properties.    -   (3) High thermal stability, in particular high melting point, to        facilitate the (normally high temperature) incorporation of the        materials into NLO devices and to provide adequate resistance to        thermal damage in the presence of laser radiation.    -   (4) Good mechanical strength.    -   (5) Lack of colour, in order to promote high optical        transmissivity at all optical wavelengths and low heat        absorption of laser energy.    -   (6) Absence of hygroscopy.    -   (7) Ability to form hydrate-free crystals, since prolonged        heating of a hydrated crystalline material by laser radiation        may promote the liberation of water vapour and so degrade the        structure of the material from within.    -   (8) High SHG response chracteristics (should generally be        superior to that of urea).    -   (9) Low cost of production.

Very few crystalline materials possess a sufficient number of theseproperties to render them useful in practical NLO devices. The principaldisadvantage of known crystalline inorganic compounds exhibiting SHGresponse is their generally low threshold to optical damage which leavesthem vulnerable to damage by laser light. The principle disadvantages ofknown crystalline organic compounds exhibiting SHG response is theirgenerally high cost of production, and their generally poor crystalstrength and high volatility which results in mechanical damage anddissipation of the materials. Any material damage results in a reductionof power in the light emitted from NLO devices employing the material,and also results in the material absorbing excessive amounts of heatwhich can cause futher damage to the material. For example, although KDPand LAP are currently widely used in NLO devices, KDP is hygroscopic,and LAP is both hydrated and possesses a melting point of only 140° C.,and neither exhibits an SHG response of high magnitude. The materialsdisclosed for use in NLO devices in the afore-mentioned U.S. patentdemonstrate an improvement over earlier SHG materials, particularly asregards their cost and effectiveness but further improvements arecontinuously being sought for such materials.

It is an object therefore of the present invention to provide novelmaterials for use in NLO devices which demonstrate enhanced propertiesin the areas mentioned above. It is a further object of the presentinvention to provide NLO devices which have enhanced performance andwhich incorporate the novel SHG responsive materials.

According to a first aspect of the present invention therefore there areprovided materials having SHG activity which comprise those morpholiniumor thiomorpholinium salts of hydroxy-substituted aliphatic or aromaticcarboxylic acids selected from tartaric acid and hydroxy-substitutedbenzoic and cinnamic acids, which exhibit a non-centrosymmetriccrystalline form. Especially preferred materials demonstrating SHGactivity include the following novel salts: morpholinium3-hydroxybenzoate, morpholinium 4-hydroxybenzoate, morpholinium3,5-dihydroxybenzoate, morpholinium 3-fluoro-4-hydroxybenzoate,dimorpholinium 2,3,5,6-tetrafluoro-4-hydroxybenzoate, morpholinium4-(4-hydroxyphenyl)benzoate, morpholinium tartrate and thiomorpholinium4-hydroxybenzoate.

All of these compounds exhibit SHG activity and have generallyfavourable properties in respect of the various criteria discussedabove. The materials morpholinium 4-hydroxybenzoate and morpholinium3,5-dihydroxybenzoate are considered to be particularly well suited touse as SHG materials in non-linear optical devices.

As will be appreciated from the reference to dimorpholinium2,3,5,6-tetrafluoro-4-hydroxybenzoate above, in certain instances wherethere are appropriate substituents on the aromatic ring, the hydroxylsubstituent may also combine with the base to form a 2:1 base:acid saltand such substances—provided that they exist in a crystalline form whichexhibits non-centrosymmetry—are also comprehended to be within the scopeof this invention.

The present invention therefore further provides, in a second aspect, anNLO device comprising a crystalline SHG material mounted in the opticalpath of a laser, wherein the crystalline material is selected from thegroup comprising morpholinium and thiomorpholinium salts ofhydroxy-substituted aliphatic or aromatic carboxylic ahcids selectedfrom tartaric acid and hydroxy-substituted benzoic and cinnamic acids,which salts exhibit a non-centrosymmetric crystalline form. Especiallypreferred salts for use in such an NLO device comprise one of the novelsalts listed above and more especially either morpholinium4-hydroxybenzoate or morpholinium 3,5-dihydroxy-benzoate.

The principal advantages of using the present materials in an NLO deviceare that they are derived from starting materials, ie. morpholine andacids, which are both relatively inexpensive and readily availablecommercially. The salts themselves are easily prepared using absoluteethanol as the solvent and advantageously are transparent andcolourless. They have melting points in excess of 160° C. and exhibitfavourable crystal growth habits and, most importantly, good SHGperformance at 1064 nm. The powder efficiency of the materials is, forexample, better than that of urea which is the reference material forSHG activity.

The invention will now be further illustrated by the following examplesshowing the preparation of SHG materials according to this invention.

Preparation of Novel Salts

EXAMPLE 1 Preparation of Morpholinium 3-hydroxybenzoate (M3)

In a conical flask, cold absolute ethanol (ca. 40 ml) was added to 99%pure 3-hydroxybenzoic acid (1.8495 g; 13.39 mmol). Dissolution of theacid was achieved at room temperature by constant stirring. Morpholine(1.1653 g, 13.39 mmol) was added dropwise to the acid solution. Thereaction was exothermic and some white fumes were observed; the turbidwhite solution of the acid turned yellow on addition of the colourlessmorpholine. The opening of the conical flask was covered with parafilmwhich was then punctured to allow slow evaporation of the solvent atroom temperature. Transparent white polyhedral crystals of morpholinium3-hydroxybenzoate were formed within 24 hrs. The crystals were filteredand washed with small aliquots of cold ethanol. Yield >70% (notoptimised).

C:H:N:—Found C, 58.8%; H, 6.8%; N, 6.2%. Calculated for 1:1 salt:—C,58.7%; H, 6.7%; N, 6.2%. Melting point: 141.8° C. Space Group Cc (No. 9)

EXAMPLE 2 Preparation of Morpholinium 4-hydroxybenzoate (M4)

In a conical flask, cold absolute ethanol (ca. 50 ml) was added to4-hydroxybenzoic acid (2.4559 g; 17.78 mmol). Dissolution took place onheating the solution on a hot plate with stirring to a maximum of 78° C.To the cooled (unfiltered) colourless solution morpholine (1.5491 g;17.78 mmol) was added dropwise by pipette. The reaction was exothermicwith an increase in temperature of ca. 15° C. on addition of themorpholine; emission of some white fumes and a slight yellowing of theacid solution occurred. Immediate precipitation of morpholinium4-hydroxybenzoate in white powdered form occurred. The powder wascollected by filtration, dissolved in the minimum of boiling ethanol togive a colourless solution and filtered. The conical flask containingthe hot solution was sealed with parafilm and surrounded in aluminiumfoil. White transparent rhombic plates were afforded within 1 hr. ofslow evaporation of the solvent at room temperature. Yield >85% (notoptimised).

C:H:N:—Found C, 58.7%; H, 6.8%; N, 6.2%. Calculated for 1:1 salt C,58.7%; H, 6.7%; N, 6.2%. Melting point: 186.7° C. Spacegroup Cc (No. 9).SHG activity was better than for urea.

EXAMPLE 3 Preparation of Morpholinium 3,5-dihydroxybenzoate (M35)

In a conical flask, covered with aluminium foil, cold absolute ethanol(ca. 50 ml) was added to 3,5-dihydroxybenzoic acid (2.7056 g; 19.62mmol). Dissolution took place on heating the solution on a hot platewith stirring to a maximum of 78° C. To the cooled, slightly beigesolution morpholine (1.5294 g; 19.62 mmol) was added dropwise bypipette. The reaction was exothermic with an increase in temperature ofca. 10° C. on addition of the morpholine; the emission of some whitefumes and a slight yellowing of the acid solution occurred. The conicalflask was kept cool at ˜5° C. and precipitation of morpholinium3,5-dihydroxybenzoate as off-white powdered form occurred within 48 hrs.of reaction. The powder was collected by filtration, and recrystallised(dark) from a minimum volume of hot isopropyl alcohol: water (10:1)mixture to give hedgehog-type clusters of transparent off-white plates.Yield >85% (not optimised).

C:H:N:—Found C, 54.7%; H, 6.3%; N, 5.8%. Calculated for 1:1 salt C,54.8%; H, 6.3%; N, 5.8%. Melting point: 218.3° C. Space Group Pna2₁ (No.33).

EXAMPLE 4 Preparation of Morpholinium Tartrate (Mtart)

Cold absolute ethanol (ca. 60 ml) was added to L-tartaric acid (2.9451g; 19.62 mmol) in a conical flask. The acid was dissolved by heating themixture on a hot plate with stirring to ca. 78° C. The colourlesssolution was cooled to below 30° C. and morpholine (1.7095 g; 19.62mmol) was added dropwise by pipette. No discoloration of the acidsolution occurred, The product morpholinium tartrate precipitated out ofthe solution immediately as a white powder. The product was collected byvacuum filtration and recrystallized from boiling ethanol. The resultingflaky thin plates were unsuitable for single crystal x-ray diffraction.The flaky crystals were recrystallized from boiling methanol to yieldwhite transparent trapezia suitable for x-ray analysis. Yield >85% (notoptimised).

C:H:N:—Found C, 40.4%; H, 6.4%; N, 5.8%. Calculated for 1:1 salt C,40.5%; H, 6.4%; N, 5.9%. Melting point: 170.6° C. Spacegroup P2₁2₁2₁(No. 19). SHG activity was similar to that of ADP.

EXAMPLE 5 Preparation of dimorpholinium2,3,5,6-tetrafluoro-4-hydroxybenzoate (M24F4)

In a conical flask, cold absolute ethanol (ca. 40 ml) was added to2,3,5,6-tetrafluoro-4-hydroxybenzoic acid (3.839 g; 16.83 mmol). Theacid was dissolved by heating on a hot plate with stirring. To thecooled, colourless solution morpholine, in double the molar ratio(2.9324 g; 33.67 mmol), was added dropwise by pipette. The solutionremained colourless. The conical flask was covered in punctured parafilmand the solvent was allowed to evaporate at room temperature. After 1 hrthe product dimorpholinium 2,3,5,6-tetrafluoro-4-hydroxybenzoate hadprecipitated out as a microcrystalline powder. Recrystallisation of thepowder from boiling ethanol afforded small transparent white cube-typecrystals. Yield >80% (not optimised).

C:H:N:—Found C, 46.6%; H, 5.1%; N, 7.2%. Calculated for 2:1 base:acidsalt C, 46.9%; H, 5.3%; N, 7.3%. Melting point: 155.0° C. Spacegroup Cc(No. 9). The SHG activity for this material was weaker than that of ureabut better than for ADP.

EXAMPLE 6 Preparation of Morpholinium 3-fluoro-4-hydroxybenzoate (M3F4)

Cold absolute ethanol (ca. 20 ml) was added to 3-fluoro-4-hydroxybenzoicacid (0.4203 g; 2.69 mmol). The solution was heated to aid dissolution.Morpholine (0.2346 ml; 2.69 mmol) was added dropwise to the cooled beigesolution of the acid. No colour change to the acid solution occurred.The solvent was allowed to evaporate slowly at room temperature. Theproduct morpholinium 3-fluoro-4-hydroxybenzoate precipitated out ofsolution within 48 hr as a beige powder. Recrystallization of the powderfrom the minimum of boiling ethanol afforded very flaky off-whiteplates. The crystals were not suitable for single crystal x-raydiffraction. Yield >85% (not optimised).

C:H:N:—Found C, 54.3%; H, 5.8%; N, 5.7%. Calculated for 1:1 salt C,54.3%; H, 5.8%; N, 5.8%. Melting point: 186.7° C. SHG activity slightlyweaker than that of urea.

EXAMPLE 7 Preparation of Morpholinium 4-(4-hydroxyphenyl)benzoate(M4long)

Absolute ethanol (ca. 40 ml) was added to 4-(4-hydroxyphenyl)benzoicacid (1.4092 g; 6.58 mmol). The mixture was heated to completedissolution. The colourless solution was allowed to cool to below 30° C.and morpholine (0.573 g; 6.58 mmol) was added dropwise. No exothermicityor discoloration of the acid solution was observed on addition of thebase. The solvent was allowed to evaporate slowly at room temperature.Within 48 hr the product morpholinium 4-(4-hydroxyphenyl)benzoate hadcrystallised out of solution as elongated transparent white rectangularplates. Yield >85% (not optimised).

C;H;N:—Found C, 68.1%; H, 6.5%; N, 4.7% Calculated for 1:1 salt C,67.8%; H, 6.4%; N, 4.7%. Melting point: 212.4° C. Spacegroup P2₁2₁2₁(No. 19). SHG activity is similar to that of urea.

EXAMPLE 8 Preparation of Morpholinium 4-hydroxycinnamate (M4HCA)

In a 500 ml conical flask cold absolute ethanol (ca. 200 ml) was addedto 4-hydroxy cinnamic acid (12.1018 g, 73.72 mmol) to give a pale yellowsolution. (Cold ethanol was used since a darkening of the acid solutionwas observed when hot ethanol was used. This is possibly due to slowoxidation of the double bond of the acid in solution, the rate of whichis increased by heating).

Morpholine (6.4224 g, 73.72 mmol) was added to the solution of4-hydroxycinnamic acid in ethanol slowly by pipette while stirring thesolution. A slight darkening of the pale yellow acid solution wasobserved on addition of the morpholine. The reaction was mildlyexothermic. The conical flask was covered in parafilm which waspunctured to allow slow evaporation of the solvent. The solution wasplaced in the refrigerator and transparent white thin conglomeratedplates of morpholinium 4-hydroxycinnamate were formed within 72 hours.The crystals were isolate by vacuum filtration and washed with smallaliquots of cold ethanol. Yield >70% (not optimised).

C;H;N:—Found C, 62.3%; H, 6.8%; N, 5.5% Calculated for 1:1 salt(C₁₃H₁₇O₄N)C 62.1%; H, 6.8%; N, 5.6%. Melting point: 179.7° C.Spacegroup Cc (No. 9). SHG activity is greater than that of urea.

EXAMPLE 9 Preparation of thiomorpholinium 4-hydroxybenzoate (TM4)

In a conical flask, cold absolute ethanol (ca. 100 ml) was added to4-hydroxybenzoic acid (5.4932 g; 39.77 mmol). The mixture was heated toafford complete dissolution. Thiomorpholine (4.104 g; 33.77 mmol) wasadded to the cooled acid solution. The thiomorpholine, which was darkyellow, caused the solution of acid to assume a yellow colouration. Nowhite fumes were observed unlike those observed during the reaction ofmorpholine and 4-hydroxybenzoic acid. The reaction was mildlyexothermic. Immediate precipitation of the product thiomorpholinium4-hydroxybenzoate as a white powder occurred. The powder was collectedby vacuum filtration, washed with cold ethanol then recrystallized fromboiling ethanol. Two polymorphs of the 1:1 salt have been isolated:white transparent cube-type crystals of the compound in acentrosymmetric space group P2₁/c (No. 14) and white transparentelongated plates of the same 1:1 salt in the noncentrosymmetric spacegroup Cc (No. 9). In some batches of the compound preferentialcrystallisation of the Cc polymorph occurred after tworecrystallizations of the salt in boiling ethanol. However, thisnoncentrosymmetric polymorph has proved to be unstable, with thecrystals becoming opaque over time (even if sealed in a container).Yield (combined polymorphs)>85% (not optimised).

C:H:N (centrosymmetric polymorph):—Found C, 54.3%; H, 6.3%; N, 5.9%Calculated for 1:1 salt: C, 54.8%; H, 6.3%; N, 5.8%. Melting points:165.6° C. (noncentrosymmetric polymorph), 191.2° C. (centrosymmetricpolymorph). Spacegroup C_(c) for the non-centrosymmetric form. SHGactivity is slightly better than for ADP.

EXAMPLE 10 Preparation of thiomorpholinium 3,5-dihydroxybenzoate (TM35)

Absolute ethanol (ca. 40 ml) was added to 3,5-dihydroxybenzoic acid(2.298 g; 14.91 mmol) in a conical flask. The mixture was heated tocomplete the dissolution. Thiomorpholine (1.539 g: 14.91 mmol) was addeddropwise to the cooled pale beige solution of the acid. Addition of thedark yellow base to the acid resulted in a bright greenish yellowcoloration. The reaction was only mildly exothermic unlike that betweenmorpholine and 3,5-dihydroxybenzoic acid. Precipitation of a beigepowder occurred after 24 hrs. Recrystallization of the compound yieldeda beige microcrystalline powder and some small rectangular plates.Analysis of the crystal structure revealed the plates to bethiomorpholinium 3,5-dihydroxybenzoate in the centrosymmetric polymorphC2/c. Further recrystallization of the compound afforded beige hexagonalplates which were found to be the 1:1 salt in the non-centrosymmetricspace group P2₁2₁2₁ (No. 19). Yield (combined polymorphs)>70% (notoptimised).

C:H:N:—(initial powder) Found C, 51.4%; H, 6.0%; N, 5.1% Calculated for1:1 salt C, 51.5%; H, 5.9%; N, 5.4%. Melting point: 180.2° C.(centrosymmetric polymorph); 184.9° C. (non-centrosymmetric polymorph).SHG activity is slightly better than for urea.

SHG Performance

Samples of all of the materials prepared as described above were firstsubjected to an initial screening with the results shown in Table 1. Forthis examination ungraded (as to size) powdered samples of each materialwere sandwiched between two glass plates which were then held in thepath of a pulsed Nd:YAG laser operating at 1064 nm. (Model SL804 fromSpectron Laser Systems Ltd). The SHG response was judged by viewing theintensity of green light generated (viewed through laser goggles whichpassed green light but stopped the fundamental 1064 nm radiation). TheSHG activity was qualitatively compared to that of a reference samplewhich was either ungraded urea or ammonium dihydrogen phosphate (ADP).

TABLE 1 Initial Screening of Products Sample Example referenceAppearance SHG Response 1 M3 Polyhedra, transparent slightly weaker thanurea white 2 M4 Polyhedra, white better than urea 3 M35 Polyhedra, whiteslightly better than urea 4 MTart White, transparent similar to ADPtrapezia 5 M24F4 White, transparent better than ADP, weaker crystalsthan urea 6 M3F4 Off-white plates slightly weaker than urea 7 M4longRectangular plates, similar to urea transparent 8 M4HCA White plates,better than urea transparent 9 TM4 Off-white powder slightly better thanADP 10 TM35 Beige hexagonal plates slightly better than ureaAssessment of Phase Matching Properties.

The phase matching properties of those more promising materials whichperformed well in the initial screening have been evaluated using thewell established Kurtz and Perry technique (J Appl Phys, 1968, 39,3798). In this method the SHG intensity (λ=532 nm) is measured as afunction of the particle size for a fixed input pulse energy. In phasematching materials, a particular direction exists in which both thefundamental wave (frequency ω) and the second harmonic wave (frequency2ω) travel in phase through the material. The consequence of this isthat the harmonic fields can build up in amplitude thus making thematerial more desirable for SHG applications.

To perform the Kurtz and Perry test powdered samples of the materialswere graded using a mechanical shaker set up with a set of standardsieves (550 μm to 40 μm). The graded powders were then mounted in a thinaluminium holder placed between two glass slides. The following criteriawas used to ensure the validity of the method:r<L<Dwhere,

-   -   r=average particle size    -   L=thickness of the powdered sample (i.e. thickness of the        aluminium spacer)    -   D=diameter of the laser beam incident on the sample.        In the present experiments, there was at least an order of        magnitude difference between these parameters.

The same laser as before was used to provide 12 ns FWHM pulses at 1064nm. The energy of the input pulse was directly measured using aScientech AD30 energy meter employing a calorimetric detector typeAC2501H, supplied by Scientech. The magnitude of the SHG signal wasmeasured as the number of counts using an Optical Multichannel Analyser(OMA) with an intensified array. This was preferred to a photomultipliertube as it enabled the presence of any fluorescence or abnormal spectralemissions from the sample, for example due to damage etc., to bedetermined.

Materials M3, M4 and M35 were selected for testing by theabove-described method and the results of these tests showed that theSHG signal level does not decrease for increasing particle size. This isan indication of phase matching condition.

The experimental set-up was validated by measurements on two standards:urea (organic standard) and an inorganic standard, ADP. Both were foundto be phase matching materials. This result is consistent with thepublished data.

Crystal Growth

a) Equipment

Crystals of M4, M35 and M3 were grown from solution by lowering thetemperature of the saturated solutions. The crystal growth equipmentconsists of a heated tank of water into which the vessel containing thesaturated solution of the material is held. The glass tank is heatedthrough a UV block filter by an infra-red lamp. The temperature iscontrolled by a mercury contact thermometer. The glass tank is coveredby aluminium foil to exclude UV in sunlight from entering the glasstank.

b) Growth of M4 crystals.

Crystals of M4 have been grown using the equipment described above, bothfrom ethanol and IPA by reduction of temperature of the saturatedsolutions (12 g of M4 in 820 ml of ethanol) from 40° C. to 25° C. A seedcrystal, which had been previously washed with a solvent was suspendedfrom a stirrer and carefully introduced into the saturated solution.After a short equilibriation period, during which the seed was observedfor any signs of dissolution, the temperature of the tank was set toreduce. Several temperature decay rates were tried. A decay of −0.5° C.per 24 hr was found to give the best results. In a typical run, acrystal of M4 20 mm×15 mm×6 mm was grown in 16 days. The weight of thecrystal was 1.9 g. The growth run was accompanied by precipitation ofmultiple seeds. Ethanol grown crystals are the largest, typically 15mm×10 mm×4 mm. IPA grown crystals are smaller, mainly due to the factthat M4 is less soluble in IPA.

Crystals grown from IPA and ethanol both show high SHG activity.

c) Growth of M35 Crystals

The solubility of M35 has been determined in isopropyl alcohol/water(10:1 ratio) mixture over a temperature range of 5° C. to 40° C. Crystalgrowth attempts using decay rates of −0.25° C. per 24 hr have yielded agood stock of small triangular shaped crystals with typical dimensionsof approximately 2 mm×2 mm×1 mm. One of these crystals was used as aseed to grow a larger crystal approximately 5 mm×5 mm×5 mm in size. Apart of this crystal (defective region) was removed and the resultingsection was used as a seed to grow a crystal that was approximately 4mm×3 mm×2 mm in size. The quality of this crystal was sufficiently goodto demonstrate high SHG activity.

The transmission spectrum of a single crystal of M35 exhibits opticaltransparency down to 350 mm. The crystals of M35, once removed from thegrowing solution are stable and do not show any deterioration inquality. The optical quality could be improved by better stirring.

d) Growth of M3 Crystals

M3 has as yet only been grown as small seed crystals. These have beenshown to exhibit SHG activity.

Doubling into the Blue

Experimental

The capability for novel materials of the invention to achieve frequencydoubling into the blue area of the spectrum was examined. The lightsource for this investigation was a MOPO (Master Oscillator PowerOscillator) model 730-10 pumped by GCR 270-10 Nd:YAG laser (supplied bySpectra Physics). The incident pulse width output from this laser rangedfrom 4 ns to 6 ns. Light of a specified wavelength was directed onto asample sandwiched between two plates in a sample holder. In order todetermine the energy which was incident on a sample for each testwavelength, the sample holder was movable in and out of the light beamand an energy meter was located behind the sample position so that thelight beam would be directed onto the meter when a sample was not inposition. The test wavelengths used were 950 nm, 850 nm, 800 nm and 780nm. The energy meter was a Scientech AD30 meter employing a calorimetricdetector type AC2501H supplied by Scientech.

Results

Samples of the M4, M3 and M35 materials were tested for frequencydoubling effect at each of the test wavelengths, the performance of eachsample being judged qualitatively by eye by examination of the frequencydoubled radiation scattered from the sample. These qualatativeassessments were compared with that of a similarly prepared sample ofurea having the same particle size as the materials of the invention.The results obtained are set out in Table 2. The M3 and M4 samples wereungraded while the M35 sample was a 212–300 μm graded sample.

TABLE 2 Frequency Doubling into the blue Wavelength Energy (nm) (mJ) M4M3 M35 950 9 Better Slightly less same or better 850 12.3 Slightly lessLess same or better 800 14.8 Slightly less Much less slightly less 78016.7 Slightly less Less better

The above results indicate that these materials could providecrystalline samples capable of frequency doubling into the blue withhigh efficiency. Phase matching is indicated due to the relatively largeparticle sizes used in both the graded (M35) and ungraded samples (M4and M3) used.

1. A material having SHG activity which comprises a morpholinium orthiomorpholinium salt of a hydroxy-substituted aliphatic or aromaticcarboxylic acid selected from tartaric acid and hydroxy-substitutedbenzoic and cinnamic acids, which salt exhibits a non-centrosymmetriccrystalline form.
 2. A material as claimed in claim 1 selected from thegroup comprising morpholinium 3-hydroxybenzoate, morpholinium4-hydroxybenzoate, morpholinium 3,5-dihydroxybenzoate, morpholinium3-fluoro-4-hydroxybenzoate, dimorpholinium2,3,5,6-tetrafluoro-4-hydroxybenzoate, morpholinium4-(4-hydroxyphenyl)benzoate, morpholinium tartrate, morpholinium4-hydroxycinnamate, thiomorpholinium 4-hydroxybenzoate andthiomorpholinium 3,5-dihydroxybenzoate.
 3. A non-linear optical devicecomprising a crystalline SHG material mounted in the optical path of alaser, wherein the crystalline material is comprised of a material asclaimed in claim
 1. 4. A non-linear optical device as claimed in claim 3wherein the crystalline material is comprised of either morpholinium4-hydroxybenzoate or morpholinium 3,5-hydroxybenzoate.